1320 Right Ln.

I'm considering a more aggressive cam than i have now (214/220 @ .050" 110 LSA 105 IC)... the cam math figures that there is -3 deg overlap at .050... using the advertised duration is 53 deg overlap.
My question is, at how many degrees of overlap do you cross from streetable, to non-streetable? I figure a good idle RPM for the street is around 700-775 RPM. Then just tune the chip so that closed loop doesn't come in until 825-850 RPM so as to avoid the computers neverending hunt for a smooth idle.

Specifically, the cam I'm really checking out is an ISKY specs out at:
225/234 @ .050, 112 LSA, 109 IC. overlap at .050 is just 5.5 deg according to the desktop dyno program, this cam makes awesome power without a lot of overlap. As a matter of fact, it makes just as much power as a cam with 12 deg overlap, with nearly the exact same torque/power curve.
I've got the files for the .dyn, .flw, and .cam for DD2000 if anyone wants to check out my motor... or have suggestions.

RB83L69

That's not a super aggressive cam or anything. I wouldn't consider it unstreetable, except maybe in a stock FI motor or LG4 or something like that, where it would be totally inappropriate.

What's the rest of the combo? Include converter & gears.

1320 Right Ln.

that's just the thing, it seems very mild due to the small overlap... but it makes almost the exact same curve as a cam with more duration and more overlap.
383 CID, Trick Flow 23* CNC heads, 9.7 compression, HSR intake, 52mm TB, open headers for track, flowmaster 40 for street.
The tranny got a 2600 stall, 3.42:1 axle ratio
I'm not actually planning on going borderline unstreetable, I was just wondering what type of cams ya'll have been getting away with while maintaining a 750-775 rpm idle and enough vacuum.

Damon

Overlap doesn't help much unless you have an open exhaust with tuned length headers/collectors. With a corked-up street style exhaust system a little less duration on a wider LSA is probably a good thing. It acts very similar to a bigger cam (intake sloses later, exhaust opens sooner) but you have a lot less overlap. Your EA/DD simulations are showing you this.

Try the same experiment with open/race exhaust, tuned headers, etc. and the bigger cam with the tighter LSA will pull ahead in power production.

RB83L69

"Overlap", in and of itself, doesn't affect how an engine runs. What it is, is the time during which the intake and exhaust valves are both open, or at least, open more than some arbitrary minimum (.050" in the case of DD). "Overlap" occurs as the exhaust valve closes, and the intake just begins to open. It is important to emissions; since during that time, the more efficient the exh system is at scavengine the cyl, it's possible for suction in the exh sys to literally suck intake charge (raw fuel) right through the cyl and into the exhaust. But, this has virtually no effect whatsoever on how the engine actually runs; it only causes a certain amount of HCs to appear in the exhaust.

All that "overlap" is good for, is as a convenient shorthand for comparing the intake and exhaust valve event timing. And, it can be VERY bad in a motor with a restrictive exhaust, because then you DON'T have that suction going on, and instead, you have pressure in the cyl; and you get BIG intake reversion.

What's actually more important to how an engine runs, in fact the single most important valve event, is the point at which the intake valve closes. In order to make an engine work better at higher RPMs, you want to close the intake as late as possible, in order to allow the inertia of the gas ruching through the intake to continue filling the cyl up until the last possible instant. On the other hand, doing that results in the piston's motioon on the compression stroke, pushing intake back out of th ecyl and into the intake tract. This is the cause (one of them anyway) for very low vacuum at low RPMs with a racing (high RPM) camshaft. To get the intake valve to close later, you use more duration (duh) and retard the cam with the timing set.

Cams are measured off of the intake lobe centerline. The exhaust operates BEFORE the intake. Therefore, as you increase the lobe separation, what you're actually doing, is opening the exh valve earlier. This also favors high RPM operation, because it gives the exhaust the maximum amount of time to leave the cyl. It also favors the idle quality in a stock motor, because the pressure in the exh sys has time to subside, before the intake opens. But it decreases the engine's peak torque, by dissipating the last little bit of cyl pressure before the engine has the chance to turn it into useful work. The overall effect is to lower the peak torque value, and to spread it out somewhat upwards in RPMs.

The cam specs you are talking about actually matches the HSR's flow characteristics, and as a consequence the RPM range it favors, fairly well. Rather than try to micro-manage "overlap" in your simulation software, concentrate on the bigger picture. The software will focus you on a detail that may or may not be significant to your situation, and cause you to outsmart yourself, if you're not careful.

Chris89GTA

iTrader: (2)

Overlap is a good thing and DOES help you out in Naturally Aspirated applications.

That cam does not have that much overlap really, and isn't what I would consider an aggressive cam. I am running the 227/233 in my sig on a 112 in a 355 w/ TFS heads and its what I would consider medium in aggressiveness. But my idea of streetable and others is totally differnet.

RB pretty much covered it in his post...

My car idles at 800 rpm w/ 14 inches of vacuum... I could have more vacuum had I gotten it on a 114lsa instead though. Brakes work great, and do not have any idle issues...